Occasionally, it pays to let a project sit a bit.
About one ball into reworking the back of the Iro jacket I was able to detect the repeat. Since Iro comes in skeins, not balls, I had not wound all the balls in the same direction. It is an even split between pulling from the inside and unwinding from the outside if I want the repeat to go in the same direction (I do).
Then I looked at the completed sleeve. Knit in the opposite way from the sweater body, of course.
That was the last thing I needed to motivate me to rip and reknit (it is 2 stitches too narrow).
Turns out my UFO was, in effect, an unstarted object (the gauge was right, at least).